Current melt-blown technology produces microfibers of plastic in which a plurality of laterally spaced, aligned hot melt strands of polymeric material are extruded downwardly and are immediately engaged by a pair of heated and pressurized, angularly colliding gas streams. The gas streams function to break up the strands into fine filamentous structures which are attenuated and thermally set for strength.
The feed stock used for melt blown procedures is typically a thermoplastic resin in the form of pellets or granules which are fed into the hopper of an extruder. The pellets are then introduced into a heated chamber of the extruder in which multiple heating zones raise the temperature of the resin above its melting point.
The screw of the extruder is usually driven by a motor which moves the resin through the heating zones and into and through a die. The die, which is also heated, raises the temperature of the resin and the chamber to a desired level, at which point, the resin is forced through a plurality of minute orifices in the face of the die. As the resin exits these minute orifices, it is contacted by a pressurized hot gas, usually air, which is forced into the apparatus through air discharge channels located on either side of the resin orifices. The hot gas attenuates the molten resin streams into fibers as the resin passes out of the orifices.
Primary air systems have, in the past, included baffles for providing uniform flows of gas at the exit end of melt-blown dies. See Lohkamp, et al., U.S. Pat. No. 3,825,379, July 23, 1974. More recently, air chambers have been bolted to the outside sides of the die body halves to provide compressed air through air discharge channels having a tortuous air passage including male air deflector blocks. See Buehning, U.S. Pat. No. 4,818,463, Apr. 4, 1989.
While in the main, such devices provide sufficient air flows at the nosepiece for attenuating fibrous films, the outboard torque-creating mounting of the air chambers has been known to cause bending moments in the air discharge channel, resulting in irregular slot width and set back spacing parameters. The tortuous path of known discharge channels takes a large toll on thermal efficiency and limits the maximum obtainable air flow of the die. The air chambers of such prior art dies are also typically not heated, which results in inconsistent thermal regulation of the air flow.
Accordingly, there is a need for a primary air system for use in connection with melt-blown dies which provides greater flow rates, thermal stability and dimensional control than currently available apparatus.